Filter centrifuges are described inter alia in U.S. Pat. No. 3,943,056 corresponding to German patent 22 60 461, Swiss patent 580,986 and the aforementioned German patent itself.
The known centrifuge of this type comprises a centrifugal drum with a coaxial filter means and a collection chamber for filtrate equipped with at least one drain located radially exterior to the filter means. A siphoning device, which temporarily prevents a gas feed into the collection chamber, is connected to the collection chamber and rotates along with it.
According to German Patent 22 60 461, the siphon device rotating with and mounted on the centrifugal drum separates the filtrate from the suspension after it has passed through the filter means and the collection chamber, flowing through the siphon while excluding gas from the filtrate path.
The filtrate arrives in a ring-like cup mounted on the outer side of the drum and is conducted away from it, e.g. by a takeoff pipe which can be swung into and out of action. The takeoff pipe acts simultaneously to adjust the fluid level in the ring-like cup. If this level drops in a region radially outside the filter means, the siphon device causes a pressure drop below the filter means relative to the pressure in the centrifugal housing.
Hence the driving force, which causes the filtration, is increased relative to that in a conventional centrifuge by a pressure difference which exists between the surfaces of the filter cake and the collection chamber and the filtration accelerates.
The pressure difference returns to its original value when the filtrate delivers the gaseous components to the collection chamber under the filter means or when the gas passes through the filter means.
After each centrifuge loading, i.e. after the filter cake has been removed from the drum, with the apparatus according to German Patent 22 60 461 gas forced into the collection chamber is forced out. The fluid is filled into the ring-like cup of the siphon device and the takeoff pipe swung out from the ring-like cup.
As a result a pressure drop relative to the filter means exists and the gas forced in is driven through the filter means.
The forcing out of the gas is however only possible when the filter cake has been removed up to a residual layer. If the filter cake is still in the drum, the resistance to removal of the gas through the filter cake is too great and also the filtration becomes very difficult in that case.